A mass spectrometer generally contains the following components:
(1) a device to introduce the sample to be analyzed (hereinafter referred to as "analyte"), such as a gas chromatograph; PA1 (2) an ionization source containing a chamber which produces ions from the analyte; PA1 (3) at least one analyzer or filter which separates the ions according to their mass-to-charge ratio; PA1 (4) a detector which measures the abundance of the ions; and PA1 (5) a data processing system that produces a mass spectrum of the analyte. PA1 (1) U.S. Pat. No. 5,629,519 discloses the use of molybdenum to form the end caps and ring electrodes in a three dimensional quadrupole ion trap. PA1 (2) U.S. Pat. No. 4,883,969 discloses the use of molybdenum to form the ion chamber containing a high-temperature plasma-type ion source, wherein molybdenum is used because of its high melting point. PA1 (3) U.S. Pat. No. 4,845,367 discloses a method and apparatus for producing ions by surface ionization by increasing the molecular energy range, and directing a beam of the substance to impinge against a solid surface with a high work function, such as clean diamond or dirty molybdenum, disposed in the vacuum chamber. PA1 (4) U.S. Pat. No. 3,423,584 discloses a mass spectrometer which includes a gas source and a molybdenum electrode, located outside of the ionization chamber.
In operation, the analyte is introduced into the ionization source containing the chamber in gaseous form and partially ionized by the ionization source. The resultant ions are then separated by their mass-to-charge ratio in the mass analyzer or filter and collected in the detector.
There are many types of ionization sources useful in mass spectrometry including electron impact, chemical ionization, fast ion or atom bombardment, field desorption, laser desorption, plasma desorption, thermospray, electrospray and inductively coupled plasma. Two of the most widely used ionization sources for analytes containing organic compounds are the electron impact (hereinafter referred to as "EI") and chemical ionization (hereinafter referred to as "CI") sources.
An EI source generally contains a heated filament giving off electrons which are accelerated toward an anode and which collide with the gaseous analyte molecules introduced into the ionization chamber. Typically, the electrons have an energy of about 70 eV and produce ions with an efficiency of less than a few percent. The total pressure within the ionization source is normally held at less than about 10.sup.-3 torr. The ions produced are extracted from the EI source with an applied electric field and generally do not collide with other molecules or surfaces from the time they are formed in the EI source until the time they are collected in the detector.
In contrast to the EI source, a CI source actually produces ions through a collision of the molecules in the analyte with primary ions present in the ionization chamber or by attachment of low energy electrons present in the chamber. A CI source operates at much higher pressures, typically from about 0.2 to about 2 torr, than the EI source operates in order to permit frequent collisions. This pressure may be attributed to the flow of a reagent gas, such as methane, isobutane, ammonia or the like, which is pumped into the chamber containing the CI source. In a typical configuration, both the reagent gas and the analyte are introduced into the chamber containing the CI source through gas-tight seals. The reagent gas and the analyte are sprayed with electrons having an energy of 50 to 300 eV from a filament through a small orifice, generally less than 1 mm in diameter. Ions formed are extracted through a small orifice, generally less than 1 mm in diameter, and introduced into the analyzer or filter. Electric fields may be applied inside the CI source, but they are usually not necessary for operation of the CI source. Ions eventually leave the CI source through a combination of diffusion and entrainment in the flow of the reagent gas.
In the chemical ionization chamber of the CI source, the pressure attributable to the analyte amounts to only a small fraction of the pressure attributable to the reagent gas. As a result, the electrons which are sprayed into the chamber preferentially ionize the reagent gas molecules through electron impact. The resulting ions collide with other reagent gas molecules, occasionally reacting to form other species of ions. These reactions can include proton transfer, additions, hydride abstractions, charge transfers and the like. Negative ions can be formed by attachment of slow electrons to analyte molecules. The positive ions, together with the primary and secondary electrons, form a plasma in the chamber.
The positive ions of the analyte are produced in multiple steps. First, positive ions of the reagent gas molecules are formed by electron impact. Subsequently, the positive ions of the reagent gas molecules are converted to other ion species (hereinafter referred to as "reagent ions") by ion-molecule reactions. The reagent ions then react with molecules in the analyte to form positive ions characteristic of the molecules in the analyte which are then analyzed.
The negative ions of the analyte are produced differently than the positive ions. The ionization plasma contains low-energy or thermal electrons which are either electrons that were used for the ionization to form the positive ions and later slowed, or electrons produced by ionization reactions. These low-energy electrons, typically in the range of 0 to about 10 eV, then react with the molecules of the analyte to form negative ions characteristic of the molecules in the analyte either through direct attachment (capture) or dissociative attachment of an electron.
In CI, the character and quantity of analyzable ions from the molecules in the analyte depend upon reactions occurring on the inner surfaces of the chamber containing the ionization source. For example, the analyte can degrade, i.e., convert to other compounds, or can simply adsorb onto the surface of the chamber and desorb at a later time. Depending upon the compound, many unexpected ions can appear as a result of the catalytic processes involving the surfaces. The result is apparent chromatographic peak-tailing, loss of sensitivity, nonlinearity, erratic performance and the like. Therefore, cleanliness is critical to the proper performance of the mass spectrometer using a CI source, particularly when performing quantitative analysis of low level materials, such as for gas chromatography/mass spectrometer analysis of pesticide residues, drug residues and metabolites, and trace analysis of organic compounds.
Efforts have been made to address sample degradation problems in the ionization chamber of a mass spectrometer by substituting or modifying the surfaces of the ionization chamber. For example, U.S. Pat. No. 5,055,678 discloses the use of a chromium or oxidized chromium surface in a sample analyzing and ionizing apparatus, such as an ion trap or ionization chamber, to prevent degradation or decomposition of a sample in contact with the surface. U.S. Pat. No. 5,633,497 discloses the use of a coating of an inert, inorganic non-metallic insulator or semiconductor material on the interior surfaces of an ion trap or ionization chamber to reduce adsorption, degradation or decomposition of a sample in contact with the surface. Furthermore, coating the inner surface of the ionization chamber with materials known for corrosion resistance or inertness, such as gold, nickel and rhodium, may improve degradation of analytes, such as pesticides, drugs and metabolites, to some degree.
Others have attempted to prevent degradation problems by treating the inner metal surfaces of the analytical apparatus with a passivating agent to hide or destroy active surface sites. For example, alkylchlorosilanes and other silynizing agents have been used to treat injectors, chromatographic columns, transfer lines and detectors in gas chromatography. Such treatments have been successful in deactivating metal surfaces and thus have prevented degradation. Unfortunately, the materials used for such treatments have a sufficiently high vapor pressure to produce organic materials in the gas phase within the volume of the ionization chamber and are ionized along with the analyte, producing a high chemical background in the mass spectrum.
Others have formed the ionization chamber with electropolished stainless steel surfaces. However, mass spectrometers using such ionization chambers have been found to give variable results and do not prevent degradation of the analyte over time.
Applicants have unexpectedly discovered that the use of molybdenum on the inner surfaces of the chemical ionization chamber of a mass spectrometer reduces the adsorption, degradation or decomposition of the analyte and reduces the adverse reactions of gaseous ions on the inner surfaces of the chamber, thereby improving the performance of the mass spectrometer.
Molybdenum has been used to construct various components of mass spectrometers. For example:
However, no one has heretofore constructed an ionization source containing a chemical ionization chamber wherein the inner surfaces of the chamber are formed from molybdenum.
In ion traps and EI sources, ions that are formed by electron impact within the ionization chamber or trap rarely interact with the surfaces of the chamber or trap. As such, it is not usually necessary to prevent adsorption, degradation or decomposition of the analyte ions or to prevent adverse reactions of gaseous ions on the surface because any such secondary ions are not detected and do not interfere with or affect the intended measurement. The degradation of concern in ion traps and EI sources is caused by modification of the neutral analyte by hot surfaces prior to electron impact. In stark contrast to the ion traps and EI sources, ions formed from the analyte in a CI source react with or on the surface of the chamber many times before they exit the chamber. Thus, the type and importance of adsorption, degradation or decomposition experienced in ion traps and EI sources differs significantly from the type and importance of adsorption, degradation or decomposition experienced in CI sources.
It has been found that solutions to the degradation problems in ion traps and EI sources, including the use of inner surfaces of the ionization chamber formed from inert materials, such as gold, nickel and rhodium; chromium and oxidized chromium; or an inert, inorganic non-metallic insulator or semiconductor material, as discussed above, do not solve the degradation problems associated with CI sources. Thus, applicants were particularly surprised to discover that the adsorption, degradation and decomposition of analyte could be reduced by using non-inert molybdenum on the inner surfaces of the chamber containing the CI source while simultaneously improving the performance of the mass spectrometer. Applicants were also surprised to discover that many catalytic reactions expected with chromium surfaces were not a problem with molybdenum surfaces.